At present, agricultural expansion into many semi-arid and arid regions is severely limited by the scarcity of good quality irrigation water and the cost of the energy necessary to provide irrigation to these areas. Generally, although water resources remain available to these regions, they are mostly unexploited due to two main factors: (1) unacceptable quality of the water for irrigation (e.g., water high in salts or toxins); and (2) the requirement of great conveyance energy (i.e., water transportation, pumping, and pressurizing energy) in order to approach economic usefulness. Due to increased competition for good quality water and higher conveyance costs, the total cost of irrigation is continually increasing in such areas. If this trend continues, not only will agricultural expansion be curtailed, but substantial reductions in irrigated agriculture may be necessary.
Another problem in these dry regions arises with regard to salt accumulation in agricultural basins. For many agricultural basins associated with irrigated areas, irrigation drainage water has become a severe environmental pollutant. This is primarily due to the leachates and tailwaters of the drainage which cause increased salinity of groundwater and surface water resources. Since water imports are limited to their present levels in most of these basins, there have been gradual increases in the proportion of impaired water resources to good quality water resources in these basins.
As a consequence of the situation described above, methods for improving irrigation management, water conservation, and saline water conversion have become increasingly active areas of water resources research. Recently, irrigation management has been improved through more precise irrigation scheduling and increased use of drainage systems. In addition, water conservation techniques such as installation of drip (trickle) irrigation systems and the adoption of drought tolerant crops have helped to maintain the present level of irrigated agriculture in semi-arid and arid regions while leading to reductions in the amount of agricultural drainage. However, when the only water resources available are saline or otherwise impaired, irrigation management and water conservation methods become ineffective, necessitating alternatives such as the use of salt tolerant crops or saline water conversion processes. Although there has been some success with a limited number of salt tolerant crops, water conversion and purification techniques are not being used extensively because of the high costs involved. At present, there are ever increasing amounts of impaired water resources in arid and semi-arid regions that cannot be used for irrigation, and the problem of water distribution in these areas is becoming a very serious one.
In light of these trends, there exists a clear need for a low-energy inexpensive process for water purification and irrigation which is designed to help solve the complex problems associated with water scarcity, increasing water conveyance costs, and regional accumulation of salts. It would be desirable, therefore, to develop a process which can cheaply and effectively purify saline or otherwise impaired water resources, and at the same time, distribute the purified water to row crops to allow them to grow in arid and semi-arid regions.
It has been known in the prior art to develop systems in which impaired water sources are used to provide water to plants by evaporating the impure water so that the contaminants are separated out, and then applying the purified, evaporated water to the plants. However, such prior art systems have not been simple and inexpensive ones which can effectively be used for row crops. One such system, described in U.S. Pat. No. 3,653,150 (Howard), employs a transparent cover which is draped flat over an area so that evaporation takes place and water which condenses on the cover falls on the ground below. This system is complex in that it requires water tube-containing side panels which are mechanically adjusted to increase or reduce pressure on the cover means, and this cover means is designed to be deployed by tractors or other vehicles using a motor-driven takeup roll. In addition, because of the flat deployment of the cover means, water which condenses falls on the ground in random patterns over the area covered, and is thus not focused on particular furrows, as would be needed to promote maximum growth of row crops. Another system, disclosed in U.S. Pat. No. 4,178,715 (Greenbaum), involves a channel culture array which is used to retain soil, and which is placed over a flow of contaminated water. In this system, moisture condenses on the underside of the array and seeps into the soil through apertured panels. This system requires a flow of contaminated water under the arrays, a liquid impermeable portion of the arrays to keep the soil away from the contaminated water, and a cooled floor undersurface to allow moisture to condense and wet the soil. It does not make use of a simple drip irrigation system in order to focus evaporated water on row crops. It is still thus needed in the art to develop a system for irrigating row crops using contaminated or saline water which is simple and inexpensive, yet which can effectively be used to provide purified water to row crops.